We use KBr in IR spectroscopy primarily because it is transparent to infrared light, allowing for accurate and high-resolution measurements of sample spectra. KBr is commonly used to prepare samples in the form of pellets, which are ideal for IR analysis due to their minimal interference with the light path and their ability to be easily manipulated in the measurement setup.
Detailed Explanation:
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Transparency to Infrared Light: KBr is transparent to infrared radiation, which is crucial for IR spectroscopy. This transparency ensures that the infrared light can pass through the sample without significant absorption, allowing for clear detection of the sample's absorption characteristics. This property is essential for obtaining spectra with sharp peaks and good intensity.
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Sample Preparation: In IR spectroscopy, the sample is often mixed with KBr and compressed into a pellet. This method is preferred because it allows for the inclusion of just the right amount of sample (typically 1% by weight) to be analyzed without blocking the infrared light path. The pellet formation process exploits the plasticity of alkali halides like KBr when subjected to pressure, forming a transparent sheet suitable for spectroscopic analysis.
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Background Measurement and Calibration: Before measuring the sample, a background measurement is performed using pure KBr. This step is crucial for calibrating the system and ensuring that any signals observed are due to the sample and not the matrix. The sample is then mixed with KBr (diluted to 0.1% to 10%) and packed into the sample plate for measurement. This technique allows for the analysis of very small sample volumes, as little as 50 to 100 ng.
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Handling of Moisture: KBr is hygroscopic, meaning it can absorb moisture from the air. This property can affect the accuracy of IR measurements if not properly managed. To mitigate this, sample preparation and pressing are often conducted in controlled environments such as gloveboxes or under vacuum conditions to prevent moisture absorption.
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Comparison with Transmission Spectra: The diffuse reflectance method used with KBr pellets involves repeated transmission of light through the sample, which can emphasize low absorption bands. To compare these spectra with traditional transmission spectra, a Kubelka-Munk transformation is applied, ensuring accurate and quantitative analysis.
In summary, KBr is used in IR spectroscopy because of its transparency to infrared light, its utility in sample preparation, and its compatibility with various spectroscopic techniques and environments. These properties make KBr an essential component in obtaining high-quality IR spectra for a wide range of samples.
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